1. Field of the Invention
The present invention relates to a reproducing device for performing reproduction from a hologram recording medium in which data is recorded with interference fringes between reference light and signal light, and the method thereof.
2. Description of the Related Art
For example, such as disclosed in Japanese Unexamined Patent Application Publication No. 2006-107663, and Japanese Unexamined Patent Application Publication No. 2007-79438, there is a hologram recording/reproducing method wherein recording of data is performed with the interference fridges between signal light and reference light, and the data recorded with the interference fridges is reproduced by irradiation of the reference light. As for the hologram recording/reproducing method, there is a so-called coaxial method wherein recording is performed by disposing the signal light and the reference light on the same axis.
Recording Employing Phase Mask
FIGS. 64, 65A, and 65B are diagrams for describing a hologram recording/reproducing technique by employing the coaxial method, wherein FIG. 64 illustrates a recording technique, and FIGS. 65A and 65B illustrate a reproducing technique.
First, in FIG. 64, at the time of recording, incident light from a light source is subjected to spatial light intensity modulation (hereafter, also simply referred to as intensity modulation) at an SLM (spatial light modulator) 101, thereby generating signal light and reference light disposed on the same axis. The SLM 101 is configured of, for example, a liquid crystal panel or the like.
At this time, the light signal is generated by subjecting the incident light to intensity modulation according to recorded data in pixel increments. Also, the reference light is generated by subjecting the incident light to intensity modulation according to a predetermined pattern.
The signal light and reference light thus generated at the SLM 101 are subjected to spatial phase modulation by a phase mask 102. As shown in the drawing, according to the phase mask 102, a random phase pattern is provided to the signal light, and a predetermined phase pattern determined beforehand is provided to the reference light.
The reason why the reference light is subjected to phase modulation is, as disclosed in Japanese Unexamined Patent Application Publication No. 2006-107663, to enable multiplex recording to a hologram recording medium.
Description will be made here for confirmation wherein multiplex recording is to perform recording so as to overlap hologram pages (increments capable of recording at a time by interference fringes between signal light and reference light), for example, such as shown in FIG. 66.
With the hologram recording/reproducing method, a hologram page (data) recorded by employing reference light having a certain phase configuration can be read out only by irradiating reference light having the same phase configuration at the time of reproduction. This point is utilized, i.e., multiplex recording of data is performed by employing reference light having a different phase configuration at the time of recording each, the reference light according to a different phase configuration is selectively irradiated at the time of reproducing each, whereby each data of which the multiplex recording was performed can be selectively read out.
Also, the reason why a random phase modulation pattern is provided to the signal light is to improve the interference efficiency between the signal light and reference light, and realize diffusion of the spectrum of the signal light, thereby suppressing DC components to realize high recording density.
As for a phase modulation pattern as to the signal light, for example, a random pattern according to a binary of “0” or “π” is set. Specifically, a random phase modulation pattern is set wherein pixels not to be subjected to phase modulation (i.e., phase=0) and pixels to be modulated by π (180 degrees) alone are set such that the number of the former pixels and the number of the latter pixels are the same.
Here, according to the intensity modulation by the SLM 101, light of which the intensity was modulated to “0” or “1” according to recorded data is generated as signal light. Such signal light is subjected to phase modulation according to “0” or “π”, thereby generating light having “−1”, “0”, or “1 (+1)” as the wavefront thereof, respectively. Specifically, when modulation according to phase “0” is provided to a pixel of which the light intensity is modulated to “1”, the amplitude thereof is “1”, and when modulation according to phase “π” is provided, the amplitude thereof is “−1”. Note that, with regard to a pixel of which the light intensity is “0”, the amplitude thereof is kept to “0” as to either modulation of a phase “0” or modulation of a phase “π”.
FIGS. 67A and 67B illustrate the difference regarding signal light and reference light in the case of no phase mask 102 being provided (FIG. 67A) and in the case of the phase mask 102 being provided (FIG. 67B) for confirmation. Note that, in FIGS. 67A and 67B, the magnitude relation of the amplitude of light according to color density is represented. Specifically, in FIG. 67A, black→white is represented by amplitude “0”→“1”, and in FIG. 67B, black →gray→white is represented by amplitude “−1”→“0”→“1 (+1)”.
Signal light is generated of which the intensity is modulated according to recorded data. Therefore, intensities (amplitudes) “0” and “1” are not necessarily disposed randomly, and consequently, occurrence of a DC component is promoted.
The above-mentioned phase pattern by the phase mask 102 is set to a random pattern. Thus, pixels of which the light intensity is “1” within the signal light output from the SLM 101 are arranged so as to be divided to amplitudes “1” and “−1” randomly (half-and-half). Thus, the pixels are divided to amplitudes “1” and “−1” randomly, spectrums can be distributed evenly on a Fourier plane (frequency plane: this case can be conceived as an image on a medium), and thus, suppression of a DC component within signal light can be realized.
Thus, if suppression of a DC component of signal light is realized, improvement of data recording density can be realized.
A DC component occurs in signal light, and thus, a recording material reacts greatly to the DC component thereof, and accordingly, multiplex recording such as shown in FIG. 66 fails to be performed. This is because more data is prevented from multiplex recording as to a portion where a DC component is recorded.
Suppression of a DC component is realized with such a random phase pattern, thereby enabling multiplex recording of data to realize high recording density.
Returning to the description above, the signal light and reference light subjected to the above-mentioned phase modulation by the phase mask 102 are both condensed by an objective lens 103, and are irradiated on a hologram recording medium HM. Thus, interference fringes (diffraction gratings, i.e., hologram) according to the signal light (recorded image) are formed on the hologram recording medium HM. That is to say, recording of data is performed with formation of the interference fringes thereof.
Subsequently, at the time of reproduction, first, as shown in FIG. 65A, reference light is generated with the spatial light modulation (intensity modulation) of the SLM 101 as to incident light. Subsequently, according to the spatial light phase modulation by the phase mask 102, the same predetermined phase pattern as at the time of recording is given to the reference light thus generated.
In FIG. 65A, the above-mentioned reference light subjected to the phase modulation by the phase mask 102 is irradiated on the hologram recording medium HM through the objective lens 103.
At this time, as described above, the same phase pattern as at the time of recording is provided to the reference light. Such reference light is irradiated on the hologram recording medium HM, as shown in FIG. 65B, thereby obtaining diffracted light according to the recorded hologram, and consequently, the diffracted light thereof is output as the reflected light from the hologram recording medium HM. That is to say, a reproduced image (reproduced light) according to the recorded data is obtained.
Subsequently, the reproduced image thus obtained is received at an image sensor 104, for example, such as a CCD (Charge Coupled Device) sensor, CMOS (Complementary Metal Oxide Semiconductor) sensor, or the like, and reproduction of the recorded data is performed based on the reception signal of the image sensor 104 thereof.
Aperture
Now, with the above-mentioned hologram recording/reproducing technique, suppression of a DC component of signal light by the phase mask 102 is realized at the time of recording, thereby realizing high recording density. Such a technique employing the phase mask 102 achieves high recording density at a plane where multiplex recording of a hologram page can be performed.
On the other hand, heretofore, a technique for realizing reduction in the size of a hologram page has been proposed as another approach for achieving high recording density.
Specifically, as shown in FIG. 68, an aperture 105 is provided so as to input the signal light (and reference light) irradiated on the hologram recording medium HM at the time of recording, and the aperture 105 allows only light of a predetermined range from the optical axis center of the signal light to be transmitted. The aperture 105 is provided in a position serving as the above-mentioned Fourier plane (i.e., the same frequency plane as the recorded plane of a hologram page as to the medium).
Reduction in the size of a hologram page to be recorded on the hologram recording medium HM can be realized by the aperture 105 provided on the Fourier plane, and consequently, high recording density at the plane of reduction in the occupied area of each hologram page on the medium can be achieved.
Note that, in the case of employing the above-mentioned technique for realizing high recording density by employing the aperture 105, as the transmission region at the aperture 105 is narrowed, reduction in the size of a hologram page can be realized, and accordingly, further high recording density is realized. However, the transmission region is thus narrowed, which is equivalent to narrowing a passage band regarding the spatial frequency of incident light (image). Specifically, as the transmission region is narrowed, only components of a low-frequency band are allowed to be passed through, and accordingly, the transmission region serves as a so-called low-pass filter.
Determination of Pixel Position, and Over-Sampling
Incidentally, with the above-mentioned hologram recording/reproducing system, in order to reproduce data of 0 and 1 included in a hologram page correctly, the correspondence relationship of the position of each data pixel (each pixel of the SLM 101) serving as a delimiter of one data bit corresponding to which position within an image signal obtained at the image sensor 104 has to be found.
At this time, if the optics are adjusted such that each pixel of the SLM 101 (data pixel) strictly corresponds to each pixel of the image sensor 104 (detector pixel) one-on-one, processing for identifying the correspondence relationship can be eliminated. Specifically, thus, in a state in which optical pixel matching is realized strictly, it becomes self-evident whether or not an image received at a certain pixel of the image sensor 104 is an image recorded through which pixel of the SLM 101, and accordingly, processing for identifying the correspondence relationship does not have to be performed in particular.
However, in reality, it is extremely difficult and impractical to realize strict pixel matching. Therefore, heretofore, based on the premise of no strict optical pixel matching being taken, signal processing for identifying each data pixel position from a readout signal by the image sensor 104 has been performed.
In the case of no strict pixel matching being taken, irradiation is performed in a state in which a data pixel deviates from a detector pixel. At this time, in a case where a ratio according to the number of data pixels: the number of detector pixels is 1:1, resolution as to the deviation of the data pixel to the detector pixel such as described above is one time, and accordingly, deviation in increments of less than a pixel fails to be handled. Therefore, with identifying processing of a data pixel position, for example, such as shown in FIG. 69, the number of pixels, and optical system of the SLM 101 and image sensor 104 are adjusted such that the image of one pixel worth of the SLM 101 is received at n pixels (n>1) on the image sensor 104.
Such a technique for performing sampling of a reproduced image such that the image of one pixel worth of the SLM 101 is received at n pixels worth of the image sensor 104 is called over-sampling.
Note that FIG. 69 exemplifies a case where the image of one pixel worth of the SLM 101 is received at four pixels worth (2×2) on the image sensor 104, and the over-sampling rate is double, but of course, the over-sampling rate is not restricted to this.
A specific example will be described of an identifying method of a data pixel position according to the related art. First, in order to perform identifying of a data pixel position, a predetermined data pattern called a sync is inserted into a hologram page (signal light) at the time of recording beforehand.
FIG. 70 illustrates an insertion example of syncs into signal light. The example in FIG. 70 illustrates a case where syncs illustrated with outline square marks in the drawing are inserted such that a predetermined interval is provided both in the vertical and horizontal directions.
The insertion position of each sync is determined with a recorded format beforehand. The entire data array within signal light including the insertion positions of the above-mentioned syncs is determined according to a recorded format. Specifically, the insertion position of each sync such as shown in FIG. 70, the number of pixels (the number of data pixels) to be inserted therebetween, and so forth are determined with a recorded format beforehand.
From this perspective, if the insertion position of each sync can be identified from the image read out by the image sensor 104 at the time of reproduction, the position of each data pixel can be estimated in accordance with the information of a recorded format.
Specifically, as for processing at the time of reproduction, first, search of the insertion position of a sync is performed from the image read out by the image sensor 104. That is to say, the position where the above-mentioned predetermined data pattern has been obtained as syncs (the position of a detector pixel) is identified of the image read out by the image sensor 104.
Subsequently, upon the insertion positions of syncs being identified, the position of each data pixel is identified in accordance with the information of a recorded format. For example, according to a recorded format, the distance from a identified sync to a data pixel to be processed (by how many pixels the pixels are separated) can be found, and accordingly, based on the information thereof, identifying of each data pixel position is performed from the information of a identified sync position.
Such identifying processing of a data pixel position is performed, whereby the position of each data pixel within the readout image can be comprehended appropriately even in a case where optical pixel matching has not been performed.
Amplitude Value Calculation by Linear Interpolation
Also, with a hologram recording/reproducing device according to the related art, after identifying of a data pixel position as described above is performed, the amplitude value at the identified data pixel position is also calculated by linear interpolation employing amplitude values around the identified data pixel position.
In general, with the reproducing system of a data storage system, interference between codes (interference between pixels with hologram recording) can be deemed as the linear superposition of the same signal properties. Accordingly, from the perspective of this premise, the amplitude value of each adjacent data pixel can be deemed to have predetermined linear relationship.
Linear interpolation processing performed here is processing for obtaining the amplitude value of a data pixel to be processed from surrounding amplitude values on the premise of such linearity.
Sampling Theorem
Incidentally, with regard to the hologram recording/reproducing method described so far, the operation of the reproducing system thereof is, as can also be understood with reference to FIG. 65A, generally equivalent to each pixel of the image sensor 104 sampling (digital sampling) the original continuous signal (analog signal) serving as a reproduced image.
According to the Nyquist sampling theorem, the original continuous signal is sampled with a clock having a higher frequency than double of the highest frequency included therein to digitize this (digital data), whereby the original analog signal can be restored accurately from the digital data through an appropriate LPF (Low Pass Filter).
In the case of the hologram recording/reproducing system, the highest frequency of the original readout signal is determined with an aperture size (the size of the transmission region of the aperture 105 shown in FIG. 68). Specifically, one-half of the aperture size becomes the highest frequency. On the other hand, sampling frequency is determined with the over-sampling rate.
Therefore, according to the Nyquist sampling theorem, in the event that the over-sampling rate is greater than the aperture size, the original signal (i.e., reproduced image) can be restored. That is to say, with the hologram recording/reproducing system, conceptually, the relation between the over-sampling rate and aperture size should be over-sampling rate>aperture size.